Dual-machine Hot Backup deployment scheme of the email system

Dual-machine Hot Standby deployment

Dual-machine Hot Standby is a backup technology for temporary server faults. Through Dual-machine Hot Standby, it can avoid long service interruptions and ensure the system's long-term and reliable services. To avoid data loss caused by server faults, enterprises use RAID and data backup technologies. However, data backup can only be used to recover the system from problems. Both hardware and software may interrupt the email service, while raid and Data Backup cannot solve the problem of service interruption.

In the event of a crash accident, the server may be interrupted for a period of time due to minor problems or powerful technical support, which may cause mail loss, for users who need to stay online without interruption, the loss of mail is equal to the loss of money, and the loss can be very small. Such users are hard to bear the service interruption. Therefore, the dual-host hot backup is required to avoid service interruption and ensure the long-term and reliable services of the system.

1)Storage array-based hot standby

 

Storage-sharing-based hot standby is the most standard solution for hot standby. In this way, two servers are used (the mail system runs on both servers at the same time ), use the disk array of the shared storage device (the data of the mail system is stored in the disk array ). The two servers can adopt different methods, such as mutual backup, master-slave, and parallel. In the course of work, the two servers will provide external services with a virtual IP address, and send service requests to one of the servers according to their work methods. At the same time, the server uses a heartbeat line (usually using a private network) to detect the working status of another server. When one server fails, the other server makes a judgment based on heartbeat detection, switches over and takes over the service. For users, this process is fully automated and completed in a short time, so as not to affect the business. Because shared storage devices are used, the two servers use the same data, which is managed by the dual-host or cluster software.

Advantages:For the sharing mode, the database is placed on the shared storage device. When a server provides services, it reads and writes directly on the storage device. When the system switches, the other server also reads data from the storage device. It can provide fast failover without any data loss.

Disadvantages:Increased investment in storage devices. This method is preferred for powerful enterprises.

 

2)Software-based hot standby

 

Software-only data can be copied to another server in real time through image software. In this way, the same data is copied to each server. If one server fails, you can switch to another server in time.

Advantages:

① Save investment.

② Unlimited distance: The two servers do not need to be limited by the length of the SCSI cable (Fiber Channel disk arrays are not limited by distance, but the investment will be much larger ). In this way, you can deploy servers more flexibly, including improving security through the distance between physical locations.

Disadvantages:

① The reliability is relatively poor, and real-time data replication between the two servers is a relatively fragile link.

② Once a server is interrupted, complicated data synchronization and recovery are required. In addition, the system is not protected during this period.

③ There is no transaction mechanism. Because the replication is performed on the file and disk layers, whether the replication is successful or not will not affect the database transaction operations. Therefore, the data is incomplete, there is a considerable risk.

The software has maintenance and unknown bugs, so we do not recommend that customers adopt this method for hot standby. Currently, most turbomail users require dual-host Hot Standby to use the shared storage device like "disk array". As of now, no repairs or maintenance is found.